As discussed with HoanNguyen, I had put my code to hide/show the navigation bar on viewWillAppear/Disappear but finally I figured out that the trick was to set the values animated. Weird, but this solved my problem and the black area is now gone:
- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
[self.navigationController setNavigationBarHidden:self.shouldHideNavBar animated:animated];
}
- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
[self.navigationController setNavigationBarHidden:!self.shouldHideNavBar animated:animated];
}